1. Field of the Invention
The present invention relates generally to the formation of heat pipe panels and more particularly to composite micro heat pipe laminates formed from a plurality of individual micro heat pipe panels and a method of forming micro heat pipe panels.
2. Description of the Related Art
Micro heat pipes are small, wickless heat pipes which have a hydraulic diameter of the same order-of-magnitude as the capillary radius of the working fluid. Liquid transport is accomplished by the formation of a meniscus of fluid in the corners of the heat pipe due to the surface tension forces of the working fluid. The cross-sectional dimension of such heat pipes is on the order of 0.01 to 0.1 inches and the length is on the order of inches.
The concept of using heat pipes for thermal management has been used in applications ranging from semiconductor devices to the leading edges of various aircraft structures. In leading edge applications, active cooling alone cannot accomodate the magnitude and local nature of the heating associated with shock interference heating on the engine cowl lip of a hypersonic vehicle. The heating can be as high as 100,000 BTU/ft.sup.2 -sec over a 0.01-inch region. The purpose of the micro heat pipe panel is to distribute the heat over a large area so it can be effectively absorbed by an alternate system, such as active cooling (e.g., forced convection). The small size of the individual heat pipes within the micro heat pipe panel also enables an increased survivability in the event of a particle penetration or puncture.
Small width heat pipe panels using the same principles as the leading edge application can also be bonded to the underside of micro chips to enhance cooling and reliability and prolong the life of the electronics. In both these applications, the micro heat pipes are essentially parallel and embedded in the panel material.
Current methods of micro heat pipe fabrication are described by Cotter in reference to silicon micromechanical devices and typically include forming channels in a substrate followed by enclosing the channels, e.g., by bonding a plate to the substrate surface. Cotter, T. P., 1984, "Principals and Prospects of Micro Heat Pipes", Proc. 5th Int'l Heat Pipe Conf., Tsukuba, Japan, pp. 329-330.
In a related method, U.S. Pat. No. 5,179,043 to Weichold et al. discloses a method of cooling integrated circuit chips using micro heat pipes. Methods of forming the heat pipes on a surface of the chips are also disclosed. A groove, e.g., rectangular or trapezoidal, is cut into a semiconductor substrate. After grooving, layers of heat conductive material are formed on the chip by vapor deposition. This material seals the grooves, leaving micro heat pipe channels of roughly triangular cross-section.
These techniques for micro heat pipe fabrication permit only a limited number of configurations of the tubes that define the micro heat pipes.
Reflecting the more conventional approach to tube formation for heat pipes, U.S. Pat. No. 5,219,020 to Akachi discloses a structure of a micro heat pipe having an elongated metallic capillary robe having an inner diameter of up to 1.2 mm. A bi-phase compressible working fluid is sealed in the capillary tube to form a loop-type flow path between alternately arranged heat receiving portions and heat radiating portions.
In another example of tube formation, Japanese Publication No. 55-99586 to Sasaki discloses manufacturing of extra fine heat pipes using extra fine glass fiber tube rather than conventional metallics. And in another example of tube formation, Japanese Publication No. 63-226595 to Nakabashi discloses a micro heat pipe comprising a capillary tube having a working fluid inside. The pipe has an inner diameter of 2-3 mm.
A leading edge structure is illustrated by U.S. Pat. No. 5,042,565 to Yuen et al. which discloses a fiber-reinforcing composite leading edge heat exchanger and a method for producing the same. The heat exchanger includes a V-shaped composite wall that houses parallel passages. The wall thickness is typically less than 0.05 inches.
As illustrated by the above examples, in situ formation of micro heat pipes results in limitations on the possible tube configuration. In addition, existing panel configurations containing embedded micro heat pipes typically have a parallel heat pipe arrangement resulting in thermal conductivity that is limited to the direction of the heat pipes. Thus, there is a need in the art for a wider variety of embedded micro heat pipe configurations and increased effective thermal conductivity beyond that achievable with an array of parallel micro heat pipes. It, therefore, is an object of this invention to provide an improved micro heat pipe panel and composite micro heat pipe laminate which can have diverse robe configurations and which can improve the effective in-plane thermal conductivity.